Recently, as information communication services have been popularized, a variety of multimedia services has appeared, and high-quality services have appeared, a demand for a wireless communication service is rapidly increasing. In order to actively cope with such a tendency, it is necessary to increase capacity of a communication system and improve reliability in data transmission. A method of increasing communication capacity in a wireless communication environment may include a method of finding a new available frequency band and a method of increasing the efficiency of a restricted resource. As the latter method, multiple-antenna transmission/reception technologies of mounting a plurality of antennas in a transceiver to additionally ensure a space for using a resource, thereby obtaining a diversity gain or transmitting data via each of the antennas in parallel to increase transmission capacity are attracting much attention and are being actively developed.
Among the multiple-antenna transmission/reception technologies, a general structure of a multiple-input multiple-output (MIMO) system based on an orthogonal frequency division multiplexing (OFDM) will now be described with reference to FIG. 1.
In a transmitter, a channel encoder 101 adds redundant bits to transmission data bits to reduce influence due to a channel or noise, a mapper 103 converts data bit information into data symbol information, a serial-to-parallel converter 105 converts data symbols into parallel data symbols to be carried in a plurality of sub-carriers, and a multiple-antenna encoder 107 converts the parallel data symbols into space-time signals. A multiple-antenna decoder 109, a parallel-to-serial converter 111, a demapper 113, and a channel decoder 115, which are included in a receiver, perform the inverse functions of the multiple-antenna encoder 107, the serial/parallel converter 105, the mapper 103, and the channel encoder 101, respectively.
In a multiple-antenna OFMD system, a variety of technologies of increasing reliability in data transmission are required. Examples of the technologies include space-time code (STC), cyclic delay diversity (CDD), antenna selection (AS), antenna hopping (AH), spatial multiplexing (SM), beamforming (BF), and precoding. Hereinafter, main technologies will be described in more detail.
The STC is a scheme for obtaining the spatial diversity gain by successively transmitting same signals through different antennas in a multiple antenna environment. The following determinant represents a basic time-space symbol used in a system having two transmitting antennas.
      1          2        ⁢      (                                        S            1                                                -                          S              2              *                                                                        S            2                                                S            1                                )  
In the above determinant, row represents antennas and column represents time slots.
The cyclic delay diversity (CDD) is to obtain a frequency diversity gain at a receiver by allowing all antennas to transmit OFDM signals at different delay values or different sizes when a system having a plurality of transmitting antennas transmits the OFDM signals. FIG. 2 illustrates a transmitter of a multiple antenna system which uses a cyclic delay diversity (CDD) scheme.
After the OFDM symbols are separately transmitted to each of the antennas through a serial-to-parallel converter and a multiple antenna encoder, they are added with a cyclic prefix (CP) for preventing interchannel interference and then transmitted to the receiver. At this time, a data sequence transmitted to the first antenna is transmitted to the receiver as it is but a data sequence transmitted to the next antenna is cyclic-delayed by a certain bit and then transmitted to the receiver.
Meanwhile, if the aforementioned cyclic delay diversity scheme is implemented in a frequency domain, the cyclic delay can be expressed by the product of phase sequences. In other words, as shown in FIG. 3, data sequences in the frequency domain are multiplied by predetermined different phase sequences (phase sequence 1 to phase sequence M) which are differently set according to the antennas, and are subjected to an inverse fast Fourier transform (IFFT), thereby being transmitted to the receiver. This is called a phase shift diversity scheme.
According to the phase shift diversity scheme, a flat fading channel can be changed to a frequency selective channel, and frequency diversity gain or frequency scheduling gain can be obtained through channel coding. In other words, as shown in FIG. 4, if a phase sequence is generated using cyclic delay of a great value in the phase shift diversity scheme, since a frequency selective period becomes short, frequency selectivity becomes high, and after all, the frequency diversity gain can be obtained through channel coding. This is mainly used in an open loop system.
Also, if a phase sequence is generated using cyclic delay of a small value in the phase shift diversity scheme, since a frequency selective period becomes long, a closed loop system allocates a resource to the most excellent channel area to obtain a frequency scheduling gain. In other words, as shown in FIG. 4, if a phase sequence is generated using cyclic delay of a small value in the phase shift diversity scheme, a certain sub-carrier area of a flat fading channel has a great channel size and other sub-carrier areas have a small channel size. In this case, if an orthogonal frequency division multiple access (OFDMA) system which allows a plurality of users transmits a signal through sub-carrier having a great channel size for each user, a signal to noise ratio (SNR) may increase.
Meanwhile, the precoding scheme includes a codebook based precoding scheme which is used when feedback information is finite in a closed loop system and a scheme for quantizing and feeding back channel information. In the codebook based precoding scheme, an index of a precoding matrix which is previously known to a transmitter/receiver is fed back to the transmitter to obtain SNR gain.
FIG. 5 illustrates the configuration of a transmitter/receiver of a multiple antenna system which uses the codebook based precoding scheme. The transmitter and the receiver have finite precoding matrixes P1 to PL. The receiver feeds back an optimal precoding matrix index l to the transmitter by using channel information, and the transmitter applies a precoding matrix corresponding to the fed-back index to transmission data X1 to XMt. Table 1 illustrates an example of the codebook which is applicable when 3-bit feedback information is used in an IEEE 802.16e system which supports a spatial multiplexing rate of 2 and has two transmission antennas.
TABLE 1Matrixindex(binary)Column1Column2000 1 0 0 1001 0.7940−0.5801 − j0.1818−0.5801 + j0.1818−0.7940010 0.7940 0.0576 − j0.6051 0.0576 + j0.6051−0.7940011 0.7941−0.2978 + j0.5298−0.2978 − j0.5298−0.7941100 0.7941 0.6038 − j0.0689 0.6038 + j0.0689−0.7941101 0.3289 0.6614 − j0.6740 0.6614 + j0.6740−0.3289110 0.5112 0.4754 + j0.7160 0.4754 − j0.7160−0.5112111 0.3289−0.8779 + j0.3481−0.8779 − j0.3481−0.3289
Meanwhile, examples of improving reliability in data transmission in a wireless communication environment include an Automatic Repeat reQuest (ARQ) scheme and a hybrid ARQ (HARQ) scheme. These schemes will now be described in detail.
An orthogonal frequency division multiplexing (OFDM) system and its similar system define resource blocks defined in a time-frequency domain and use the resource blocks as a single unit. In a downlink, a base station allocates at least one resource block to a specific user equipment in accordance with a given scheduling rule and transmits data through a corresponding resource block. Also, in an uplink, if the base station selects a specific user equipment in accordance with a given scheduling rule and allocates a resource block to the corresponding user equipment, the corresponding user equipment transmits data to the base station through the allocated resource block. At this time, if frame loss or damage occurs in the data transmitted to the downlink or the uplink, the ARQ or the HARQ is used to correct corresponding errors.
Examples of the HARQ scheme include channel-adaptive HARQ/channel-non-adaptive HARQ and chase combining scheme/incremental redundancy scheme. In the channel-non-adaptive HARQ, frame modulation or the number of available resource blocks for retransmission is performed as it is determined during initial transmission. The channel-adaptive HARQ varies the above parameters depending on the current channel status. For example, according to the channel-non-adaptive HARQ, if a transmitting side transmits data by using eight resource blocks in case of initial transmission, the transmitting side retransmits the data by using eight resource blocks even in case of retransmission. According to the channel-adaptive HARQ, even though the transmitting side transmits data by using eight resource blocks in case of initial transmission, the transmitting side retransmits the data by using resource blocks more than or less than eight resource blocks depending on the channel status.
Furthermore, the HARQ scheme can be classified into a chase combining scheme and an incremental redundancy scheme depending on which packet is transmitted during retransmission. According to the chase combining scheme, as shown in FIG. 6, the transmitting side retransmits a packet having the same format as that used for initial transmission or same data symbols in different formats during second or third transmission if errors occur in the packet used for the initial transmission. The HARQ scheme is similar to the ARQ scheme in that the receiving side transmits NCK message to the transmitting side if the receiving side cannot demodulate a packet. However, the HARQ scheme is different from the ARQ scheme in that the receiving side stores a frame which is previously received in a buffer for a certain time period and if a corresponding frame is retransmitted, combines the retransmitted frame with the previously received frame to improve a receiving success rate. The incremental redundancy scheme is different from the chase combining scheme in that a packet having a format different from that of the packet used for initial transmission can be retransmitted. In other words, as shown in FIG. 7, additional parity part of a channel code is only retransmitted during the second or third retransmission to reduce a channel coding rate, thereby correcting packet errors.
In addition, the HARQ scheme can be classified into synchronous HARQ and asynchronous HARQ depending on whether retransmission performed after transmission failure of initial data is performed in accordance with a given timing.
Since the aforementioned multiple antenna related scheme and the ARQ related schemes have been developed independently, synergy effect according to combination of the schemes have not been obtained. In this regard, a time-space symbol based HARQ has been suggested. The time-space symbol based HARQ is used in a multiple antenna system. According to the time-space symbol based HARQ, as shown in FIG. 8, a data transmission rate increases through a bell labs layered space time (BLAST) scheme during initial transmission, and if errors occur in symbols S1 and S2 of a specific time slot, a time-space symbol is applied to the symbols of the corresponding time slot and then retransmission is performed to improve transmission reliability.
However, the aforementioned time-space symbol based HARQ has several problems. First, the time-space symbol based HARQ has limitation in that it is based on a flat fading channel whose change speed is relatively slow. Second, if multiple codewords are used, it is inefficient in that retransmission of all codewords is required even though errors occur only in some of the codewords. Third, flexibility is deteriorated in that initial transmission should be performed by a spatial multiplexing scheme such as BLAST. Finally, since the adaptive ARQ such as incremental redundancy cannot be used for the time-space based HARQ, efficient error correction cannot be performed.